The burning of fuel to produce energy, and particularly mechanical energy, is at the root of modern society. Improvement in the efficiency of such combustion, or in reduction of the emissions created by combustion, are therefore important. A variety of prime movers or engine types are currently in use. The most widespread of these are the internal combustion engine and the turbine.
The internal combustion engine, especially the spark-fired xe2x80x9cOtto cyclexe2x80x9d engine, is particularly ubiquitous, but presents significant challenges in the further improvement of its efficiency. The reciprocating piston Otto cycle engine is in principle extremely efficient. For example, an Otto cycle engine operating with a 10:1 compression ratio, constant volume TDC, no heat loss, and at constant specific heat ratio (K) should, in theory, have about a 60% cycle efficiency. However in actual practice, engines typically operate at about half these air cycle values (i.e. about 31-32% efficiency). This is due to a number of reasons, including the fact that as the fuel burns, raising air temperature, the combustion chemistry limits peak temperature through dissociation and specific heat increase. Also, heat loss, finite burning, and exhaust time requirements reduce efficiency to about 85% theoretical fuel-air cycle values. Finally, engine friction, parasitic losses, etc., reduce actual power output by another 15% or so in a naturally aspirated engine.
It is well-known that it would be more efficient to run such an engine leanerxe2x80x94i.e., at a higher stiochiometric ratio of oxygen to fuelxe2x80x94to improve efficiency and reduce NOx (nitrogen oxide) emission. However, lean burning makes it difficult to sustain flame-speed (and thus avoid misfire) in a conventional Otto cycle engine, which limits the effectiveness of this approach. This problem could be overcome to some extent by xe2x80x9csuperchargingxe2x80x9d the enginexe2x80x94i.e. running it at an inlet pressure significantly above atmospheric pressurexe2x80x94but then the problem of premature detonation must be avoided, which limits the maximum available compression ratio, and thereby decreases the efficiency.
Moreover, each improvement in compression and leanness tends to increase the creation of NOx at a given peak temperature, which must then be removed by parasitic devices, such as exhaust emission systems. Further, the exhaust emission catalysts tend to be made inefficient, or poisoned entirely, by excess oxygen.
It has been discovered that the methods described herein can be used to increase the efficiency of energy producing systems, particularly engines, and more particularly the Otto cycle engine. The modifications to present practice to achieve the improved process are relatively straightforward and easily implemented, and produce significant and synergistic effects when used in combination.
In one embodiment, a combustion engine power system comprises a combustion chamber which burns a fuel with a pressurized mixture of steam and air to produce useful power, waste heat, and a steam-containing exhaust stream; a compressor which pressurizes air to produce a pressurized air stream; a water supply containing water that is heated by waste heat from the power system and evaporated into the pressurized air stream to produce the pressurized mixture of air and steam; a expander which is driven by the steam-containing exhaust stream to produce a power output in excess of the power required to pressurize the air; and a power take-off of the excess power from the expander. In one aspect, the present power generating system in effect superimposes a Rankine or steam cycle power addition onto a conventional turbo-compressor bottoming recuperation cycle. The steam cycle uses waste heat from the engine while simultaneously diluting the working fluid (e.g. air) of the engine. This combination of the cycles (the xe2x80x9cjoint cyclexe2x80x9d) improves cycle efficiency, suppresses detonation via steam dilution, and increases engine specific power. In certain embodiments, the power system uses hydrogen to support flame propagation of the steam-diluted fuel-air mixture, and the hydrogen may be advantageously provided by reforming a fuel using the high thermal mass steam-laden engine exhaust.
According to one aspect, the Otto cycle power system of the present invention operates with a steam-diluted fuel-air charge at an elevated pressure. The working fluid of the engine (e.g. air) is compressed to a high-pressure by a compressor. The preferred pressure is in the range of about 2 to about 6 atmospheres, including pressures within this range such as 2 to 3, 3 to 4, 4 to 5, and 5 to 6 atm. One embodiment described herein uses a 4 atm pressurized air stream (1 atm=1 bar; 1 bar is approximately 0.1 megapascal (MPa)).
Then, waste heat from the power system (such as from the engine exhaust or the engine cooling system) is used to evaporate water into the pressurized air to produce a pressurized mixture of air and steam. This may be efficiently done by partial pressure boiling of water (warmed by waste heat of the engine) in the presence of the pressurized air stream at one or several locations in the system.
The pressurized steam-air mixture is then inducted into the combustion chamber of the engine, together with an appropriate amount of fuel, where they are combusted in the conventional fashion (i.e. two cycle or preferably four cycle for maximum efficiency). The water (i.e. steam) concentration in the inlet stream of the combustion chamber should be as high as practical. In a 4 atm system, this can be about 8 moles of water per mole of methane (or equivalent in gasoline).
One advantage in using a steam-diluted fuel-air mixture is a reduction in peak cycle temperature, which has the effect of improving cycle efficiency while also reducing NOx emissions. Another important advantage of operating dilute is the tremendous detonation suppression resulting from the added steam. This makes it possible to operate the engine at high pressures (e.g. 4 atm). This turbocharging of the engine inlet not only aids in burning speed, but also provides the means for hybrid power/efficiency gains, and increases engine output and mechanical efficiency well over that of the natural aspired stochiometrically correct standard engine practice.
Where the addition of steam diluent hampers the ability of the fuel mixture to burn in the engine, any conventional means for igniting a dilute fuel-air mixture may be employed. In one embodiment, the primary fuel injected into the combustion chamber is supplemented by the addition of a second fuel, such as hydrogen, to help sustain flame-front propagation in the steam-diluted mixture. Moreover, by turbocharging the engine, the resultant high-temperature and high-pressure exhaust can be advantageously used as a source of heat and/or steam to partially reform the primary fuel to provide a source of the supplemental fuel (e.g. hydrogen). Because the exhaust contains a substantial amount of steam, the exhaust itself can provide steam required for the reforming reaction. Alternatively, or in addition, steam from elsewhere in the system, such as a dedicated boiler, can be used.
The combustion in one or more combustion chambers (or cylinders) provides the primary output power of the system, and is typically used directly for mechanical work, or indirectly for electricity generation. The engine combustion also generates waste heat, some of which is contained in the high-temperature engine exhaust, and some of which is removed from the engine via a cooling fluid which circulates through the engine. Much of this waste heat, such as heat from the engine cooling loop and heat from low-temperature exhaust, is low-grade heat that is notoriously difficult to recapture in a useful manner. Consequently, in a conventional engine, this low-temperature waste heat is typically rejected from the engine.
In the present invention, however, at least a portion of this low-temperature waste heat is advantageously recaptured by using the energy of the waste heat to evaporate water into the pressurized engine oxidant (e.g. air) to produce a pressurized steam-air stream having a significant expansion potential. This expansion potential can be used to produce additional mechanical energy, and thereby improve engine efficiency, as described below. In general, as much warm water should be evaporated to recover its latent heat as can be accommodated by the pressurized air. The proportion of the latent heat that is recovered as steam depends on the type of system and on its details. A proportion of at least about 50% is desirable, and generally obtainable. With a typical Otto cycle engine, recovery in the range of about 50% to 75% is often obtainable. Recoveries significantly below 50%, for example below about 25%, while still beneficial in terms of efficiency, may not be sufficient to justify the extra cost in constructing the system of the invention.
After combustion, the exhaust stream from the combustion chamber is at a high-temperature (e.g. 2100xc2x0 Rankin, or about 1200xc2x0 K.) and is still at the elevated system pressure (e.g. 4 atm). The exhaust is loaded with steam, and has a substantial expansion potential that can be advantageously utilized to drive an expander, (preferably a turbine but not limited thereto) to produce a power output. A power take-off from the expander can be utilized, for example, to drive an electrical generator, or to gear the expander power output into the primary power output from the engine. The expander can also be coupled to and used to directly drive the air-input compressor.
In contrast to conventional turbo-compressor (Brayton) cycle engines, the present invention is able to generate significant excess power by the expansion of steam-laden engine exhaust. The steam provides an additional mass flow through the expander, for example, twice the xe2x80x9cspecific mass flowxe2x80x9d (i.e. specific-heat adjusted mass flow) of the air alone. In effect, the present invention adds a Rankine, or steam cycle, power addition to the conventional turbo-compressor bottoming recuperation cycle. Thus, instead of simply recouping the power expended in compressing the air, the xe2x80x9cjointxe2x80x9d Brayton/Rankine cycle of the present invention is able to generate significant additional power. In a 4 atm. system, for example, the expander can produce over three times, and in some cases over four times, the power that is required to drive the compressor. This excess power can be significant in terms of overall system efficiency, and can amount to a 33% increase in net power output of the system as a whole.
Moreover, this excess power of the turbine can be obtained at little or no cost, as it is derived from the recovery of low temperature xe2x80x9cwastexe2x80x9d heat via evaporation of warm water into pressurized air (i.e. the xe2x80x9cpartial pressure effectxe2x80x9d). The energy gained is essentially the latent heat consumed to vaporize water. The latent heat is a significant quantity: it takes about 2326 joules per gram to evaporate water at 60xc2x0 C., while it takes only about an additional 1465 joules per gram to heat the evaporated water (steam) by an additional 800xc2x0 C. The sequence of pressurization of air before evaporation of water is important in order to maximize efficiency improvements, because while significant energy is expended to compress the air, very little energy is required to compress the warm water to the same pressure.
A typical range of concentration of steam in the system exhaust is in the range of about 30 to 60% by weight, preferably in the range of about 30% to 50% by weight, and even more preferably in the range of about 33% to 45% by weight (for example, about 520 lbs of steam in 720 lbs of air, or about 40%). A lower end of the range is typically about 20 to 25%, which is both the general range in which the presence of steam in the pressurized fuel-air mixture begins to require the presence of hydrogen (or similar means) for reliable ignition, and about the lower limit at which the extra complexity of the xe2x80x9cjoint cyclexe2x80x9d engine is repaid by improvements in efficiency. Steam concentrations above 50% are desirable when they can be readily obtained. At very high levels of steam, such as about 75% by weight and above, the combustion of the fuel-steam-air mixture can become more difficult, and the loss in power becomes a limiting factor on maximum percent of steam incorporated, with the precise limit depending on the details of the system design.
After expansion, the expanded and cooled exhaust can next be used to provide heat to evaporate or preheat water. When sufficiently cool, it is passed through a condensing radiator to condense water. The recovered water is then recycled to provide water for making steam. The condensing radiator is optionally combined with the radiator used for cooling the engine, after the engine cooling fluid has likewise been used to evaporate or preheat water. Stoichiometric operation of the engine maximizes the condensing exhaust dewpoint. In order to maintain sufficient water recovery levels in varying ambient temperatures and climatic conditions, the exhaust dewpoint can be adjusted by selectively applying a backpressure to the exhaust (e.g. via a flow-restricting variable valve) as needed.
According to one aspect, heat from the high-temperature engine exhaust can be used to partially reform gasoline or other fuel, preferably in the presence of steam, to produce a mixture of hydrogen and combustible carbon-containing materials. Heat required for the reforming reaction is preferably provided by heat exchange with the highest temperature exhaust gases (i.e. immediately or soon after the exhaust leaves the combustion chamber). Steam, if required for the reforming process, may be obtained by injecting steam from a steam source, or by using a portion of the steam-laden exhaust itself as a steam source. Optionally, oxygen can be injected at this stage as well. When the steam-laden exhaust itself is used for the reforming, the portion of the exhaust required will vary according to the exact design and could be in the range of about 35% to about 5%, depending on the steam content of the exhaust. About 10% is optimal for the 4 atm. supercharge.
In one embodiment, the steam for the fuel reforming reaction can be made by boiling water using heat from the exhaust at a cooler portion of the exhaust stream (e.g. below the expander). Even after the exhaust stream is expanded and cooled by the expander, there is still enough heat remaining to boil some steam undiluted at atmospheric pressure. This steam, along with the fuel to be reformed, can then be supplied to a reforming zone that is heated by the high-temperature exhaust (i.e. before expansion) to support the endothermic fuel reforming reaction.
The hydrogen-containing reformate generated from the exhaust can be advantageously supplied to the combustion chamber by passing all of the fuel through the reformer, without necessarily reforming all of the fuel completely. Alternatively, a reformate can be used as a supplement to the primary fuel source, which generally comprises partially reformed and/or unreformed fuel. The presence of the hydrogen in the fuel mixture allows sufficient flame speed to support the lean, dilute combustion described above. It may be less important to supply a hydrogen fuel charge for other types of combustion. Reforming the fuel by steam reforming (reaction of fuel with water to produce hydrogen and other products)xe2x80x94including variant forms of autothermal reforming (ATR) and partial oxidation (POx)xe2x80x94is preferred. Formation of hydrogen by simple heating of fuel (xe2x80x9ccrackingxe2x80x9d) is known, and is also useable in the invention wherever the tendency to produce carbon deposits can be controlled. In principle, a store of pure hydrogen or of hydrogen mixed with another gas could also be used, although it would be less practical in most applications.
The systems and methods described herein can advantageously be used to provide a combustion engine characterized by high-efficiency and low emissions. For example, employing the principles of the present invention, a standard off-the-shelf Otto-cycle engine can perform at increased specific power with a nominal 52% efficiency, while at the same time having only trace emissions.